Adding function to sum values in custom field

If I put this portion of the above code into say single.php it works.

total_hours=0;
        $meta_key = 'session_duration';//set this to your custom field meta key
        $allhours=$wpdb->get_col($wpdb->prepare("SELECT meta_value FROM $wpdb->postmeta WHERE meta_key = %s", $meta_key));
        foreach ($allhours as $hours) {
                $total_hours = $total_hours + $hours;
        }
        echo 'Total hours are '.$total_hours;

Thanks Sam. I am now able to get the code to work, but another error is occurring.

The sum is not matching the data. It seems that data is sometimes double counted, or if changed, not deleting / overwriting but appending.

Here is my code that has been added to functions.php in my twenty eleven theme.

function total_hours(){
    global $wpdb;
    unset($hours);
    $hours = array();
    $meta_key = 'session_duration';//set this to your custom field meta key
    $hours = $wpdb->get_col($wpdb->prepare("SELECT meta_value FROM $wpdb->postmeta WHERE meta_key = %s", $meta_key));
    return array_sum($hours);
}
add_shortcode('total-hours','total_hours’);

Here is an example of the performance I am seeing with the short code [total-hours]

989.25 and then I add a custom field entry of 10, and save. The new total-hours = 1009.25. Now, I remove the 10 hours field entry, and the total hours becomes 999.25. It double counts on the initial entry, but removes the correct amount. Meaning the total becomes more and more incorrect with each entry and or change.

Well its a lot, hopefully I did it correctly.

Here’s the modified code:

function total_hours(){
    global $wpdb;
    unset($hours);
    $hours = array();
    $meta_key = 'session_duration';//set this to your custom field meta key
    $hours = $wpdb->get_col($wpdb->prepare("SELECT meta_value FROM $wpdb->postmeta WHERE meta_key = %s", $meta_key));
/*   return array_sum($hours);*/
    return  var_dump($hours);
}
add_shortcode('total-hours','total_hours');

array(376) { [0]=> string(1) "6" [1]=> string(2) "10" [2]=> string(1) "4" [3]=> string(1) "4" [4]=> string(1) "3" [5]=> string(1) "4" [6]=> string(3) "2.5" [7]=> string(3) "2.5" [8]=> string(1) "3" [9]=> string(1) "3" [10]=> string(1) "0" [11]=> string(1) "0" [12]=> string(1) "0" [13]=> string(1) "0" [14]=> string(1) "0" [15]=> string(1) "0" [16]=> string(1) "2" [17]=> string(1) "2" [18]=> string(1) "0" [19]=> string(1) "0" [20]=> string(1) "5" [21]=> string(1) "5" [22]=> string(1) "0" [23]=> string(1) "1" [24]=> string(1) "1" [25]=> string(1) "0" [26]=> string(1) "0" [27]=> string(1) "0" [28]=> string(1) "2" [29]=> string(1) "2" [30]=> string(1) "0" [31]=>

The extra lines are from me being sloppy with my code insert… added some extra line breaks to keep myself straight.